Optical Coherence Tomography Protocol

Optical coherence tomography (OCT) is an imaging technique that produces high resolution cross sectional images of optical reflectivity (1,2). It is based on the principle of low-coherence interferometry where distance information concerning various ocular structures is extracted from time delays of reflected signals. In OCT, light waves emitted by a superluminescent diode operating at 840 nm and between 200 uw and 1 mw are used to determine the images. This is analogous to the application of sound waves in B-scan ultrasonography or x-rays in computed tomography (CT). The use of light waves enables OCT to achieve an axial resolution of 10 um
(2). Lateral resolution is approximately 70 um.

As a result of this high level of resolution, OCT is particularly suitable for retinal thickness measurements. OCT images can be presented as either cross sectional images or as topographic maps. Cross-sectional or B-mode imaging is accomplished by acquiring a sequence of 100 interferometric A-scans across a section of retina. To facilitate interpretation a false color scheme is added in which bright colors such as red and white correspond to highly reflective areas and darker colors such as blue and black correspond to areas of lower reflectivity (1).

Cross-sectional images take advantage of the well defined boundaries in optical reflectivity at both the inner and outer margins of the neurosensory retina allowing for retinal thickness measurement. Retinal thickness can then be assessed longitudinally using serial OCT images. The presence of cystic spaces can be detected by the presence of focal areas of very low reflectivity within thickened neurosensory retina.

OCT images that display retinal thickness in the macula, topographically, can be produced. Such topographic maps allow determination of interpolated retinal thickness in 9 regions of the macula delineated by a standard Early Treatment Diabetic Retinopathy Study (ETDRS) grid. Topographic maps are produced by obtaining six consecutive cross-sectional scans at equally spaced angular orientations (30 degrees) in a radial spoke pattern centered on the fovea (3). Each cross-sectional scan is oriented such that each scan intersects the central fovea. Therefore, retinal thickness measurements are performed with a total of 600 points along 6 intersecting lines with 6 measurements located in the central fovea. This pattern is, in theory, advantageous as the greatest concentration of measurements is within the central fovea where accurate measurements may be most important.

Topographic maps obtained by OCT are displayed by a false-color scheme to facilitate interpretation. For cross-sectional images, bright colors correspond to areas of high reflectivity while darker colors correspond to areas of low reflectivity. For topographic maps, bright colors are assigned to areas with increased retinal thickening and darker colors are assigned to areas with less retinal thickness.

Retinal thickness is converted to a false color value for each of the 600 points measured within 3,000 microns from the center. Interpolation of polar coordinates is performed to estimate thickness in the wedge-shaped areas between each cross-sectional scan. To further facilitate interpretation, the macula is divided into 9 ETDRS regions with a central circle of 500 um radius. Two outer circles with radii of 1,500 um and 3,000 um complete the display. Nine regions are thus present and an interpolated retinal thickness is reported for each of the nine regions. A mean +/- standard deviation for the central foveal thickness is recorded for the six cross-sectional measurements through the fovea as an estimate of reproducibility.

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